1. Experiments were performed on pregnant guinea‐pigs with gestations of about 44 days to term (65 days). Under anaesthesia, one fetus in each litter was given a volume equal to 1.31% of estimated body weight (v/w) of a 40% solution of 60k‐90k dextran in Ringer solution by I.P. injection. One control litter‐mate received the same amount of Ringer solution, but without dextran. Other litter‐mates served as non‐injected controls. 2. At 20.4 (+/‐ 4.7 S.D.) hr later, the sow was re‐anaesthetized and all fetuses were removed and weighed. They were then dried to constant weight. Wet weights and dry weights were corrected for the weights of the injected materials. 3. Wet weights were normalized by dry weights to account for differences in body size. The average normalized wet weight of the fetuses injected with the dextran solution was 10.9% greater than that of the injected controls and 16.0% greater than that of the non‐injected controls. 4. The results were not affected by the removal of all amniotic fluid at the time of the injections. 5. The results could not have been due to non‐colloid osmotic forces. The Ringer solution was slightly hypo‐osmotic compared to guinea‐pig plasma and the dextran contained negligible amounts of non‐colloids. 6. The applied colloid osmotic pressure of less than 20 torr caused the transfer of a physiologically significant volume of water in a period of 1 day.
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